Thermal Shock Resistance Properties Research Articles

Thermal shock resistance properties of refractory castables bonded with a CaO-free binder

The potential of hydratable magnesium carboxylate compounds (HMC) combined with MgO powder as a binder of castables was evaluated. The green mechanical and thermal shock resistance properties of HMC-bonded castables (HMCC) and calcium aluminate cement-bonded castables (CACC) were compared. The green mechanical strength of the HMCC reached the mechanical strength of the CACC. The residual strength ratio of the HMCC is higher than that of the CACC and increases first and then decreases with the MgO/HMC ratio. The residual strength ratio's highest value is 56.8% (MgO/HMC ratio is 2/3), which is 1.8 times that of CACC. Interestingly, the microcracks in the HMCC after thermal shock are radial, while those in CACC are linear. The micromechanical testing on the matrix of castables after thermal shock showed that the ratio of the energy for crack initiation to the work of fracture of the HMCC is lower than that of the CACC, but its fracture toughness is higher than that of the CACC, indicating that the microcracks in the HMCC initiate easily but propagate hardly.

Microstructure and properties of MgO–C refractory with different carbon contents

Magnesia-carbon (MgO–C) refractory is considered as the most crucial lining material in steelmaking furnaces. To provide reference for producing high performance MgO–C refractory containing different carbon contents, the microstructure, mechanical strength, thermal shock resistance and thermal properties of this material with 1–18 wt% flake graphite were studied systematically in this work. The results showed that carbon content had small effect on microstructural evolution of MgO–C refractory after high temperature treatment, mainly MgAl2O4 particles, Al4C3 whiskers and plate-/whisker-shaped AlN formed in the matrix of all specimens. As carbon content added increased gradually, fracture strength of specimens exhibited a reduced tendency whereas the corresponding fracture displacements became higher and higher owing mainly to good sliding ability of flake graphite. Furthermore, the increase in carbon content reduced thermal expansion rate of specimens and correspondingly increased thermal conductivity, consequently contributing to enhanced resistance to thermal shock of MgO–C refractory.

Rare-Earth Oxide-Doped Magnesium Aluminate Spinel - an Overview

Abstract: Magnesium aluminate spinel (MAS) is an important refractory material known for its high refractoriness, high thermal shock resistance, high-temperature mechanical and thermal properties, high resistance to chemical attack, superior hydration resistance, etc. MAS has been employed to replace traditional chromite-based refractories in cement rotary kilns and steel ladles because chromite based refractories pose environmental issues. Thin films of MAS have potential use in thermal barrier coatings (TBCs) for the blades and vanes in the hot section components of gas turbines. Rare-earth oxide dopants play a significant role in the formation and sintering of MAS. Research has shown that the microstructure of MAS can be tailored with the help of rare-earth dopants, leading to a significant improvement in their sintering behaviour and mechanical properties. This review paper attempts to present the sintering aspects, and the mechanical and thermo-mechanical behaviour of rare-earth oxide-doped MAS.

Effects of Zr and chopped C fiber on microstructure and mechanical properties of SiBCN ceramics

To further improve the thermal shock resistance and ablation resistance of SiBCN system ceramics targeted much harsh environment, both ultra-high temperature ceramic phases and carbon fibers are expected to be incorporated simultaneously as the reinforcements. However, tough problems like difficulty in densification as well as degradation of C fibers (Cf) due to interfacial reaction usually cannot be avoided. Thus, in this study, Zr as well as chopped Cf with BN coating were introduced into SiBCN ceramics by mechanical alloying and after hot-pressing sintering to prepare C/SiBCNZr composites. The structure and phase component development have been characterized in detail by XRD, SEM, TEM and XPS etc., and mechanical properties, fracture behavior and toughening mechanisms were also investigated. Zr-B and Zr-O bonds are detected besides Si-C, N-B and N-B-C bonds when pre-alloyed ZrB2 amorphous powder was incorporated into SiBCN system by further mechanical-alloying treatment. After hot-pressing, the ultra-high temperature phases of ZrB2 and ZrN were formed as expected besides the original matrix phases of BN(C) and SiC. Excellent mechanical properties can be obtained for SiBCNZr ceramic. The BN coating on carbon fibers successfully avoids the severe fiber degradation and makes it possible for moderating strong interface bonding, which ensuring the debonding, crack deflection, fiber pulling-out and bridging during the fracture. As a result, the C/SiBCNZr composites retain good mechanical properties and fractures in a pseudoplastic fracture manner, showing better potential applications in harsh environments. Optimization of the BN coating on Cf and Zr incorporation by pre-mechanical alloying can be a good route to achieve Cf/SiBCNZr composites with a better balance between thermal shock resistance, ablation resistance and mechanical properties.

Thermal shock fatigue behaviors of various W-0.5 wt%ZrC materials under repetitive transient heat loads

Thermal shock fatigue resistance behaviors of a series of rolled W-0.5 wt%ZrC materials (WZC) fabricated using tungsten powders with different powder grain sizes of 0.2 μm (0.2WZC), 0.5 μm (0.5WZC) and 2.8 μm (2.8WZC) were investigated by simulating the effects of edge-localized modes in fusion devices like transient heat events on these WZC specimens. The as prepared three different specimens exhibit different cracking thresholds and cracking patterns under repetitive (100 shots) transient heat loads. The cracking thresholds of the as-rolled 0.2WZC and 0.5WZC are in the range of 0.22–0.33 GW/m2, but <0.22 GW/m2 for the case of 2.8WZC. Moreover, recrystallization reduces the thermal shock fatigue resistance and leads to the decrease of cracking thresholds and the increase of cracking densities. Tensile results show that both along the rolling and transverse directions, the as-rolled 0.2WZC and 0.5WZC have the comparable ultimate tensile strengths (UTS) in a relatively high level, but the as-rolled 2.8WZC has smaller ones. The correlation between the thermal shock resistance, microstructures and tensile properties has been proposed.

Effect of Different Crystal Form Nanometre Zirconia on Thermal Shock Resistance of Magnesia Ceramic

The magnesia ceramic specimens were prepared using high purity magnesia powder, monoclinic nanometre zirconia, tetragonal nanometre zirconia, cubic nanometre zirconia as starting materials in order to improve thermal shock resistance of magnesia ceramic. The specimens were sintered at 1550°C for 2 hours under air atmosphere. The thermal shock resistance and sintering property of magnesia ceramics with different crystal nanometre zirconia additives were investigated. The results show that density of the specimens with zirconia is improved due to the dispersal distribution of zirconia in periclase grain boundary. The sintering property of magnesia with zirconia additives are considerably promoted. The thermal shock resistance of the specimens with tetragonal zirconia or cubic zirconia is improved due to second crystalline phase toughening mechanism of zirconia.Thermal shock resistance of specimens with monoclinic zirconia is improved due to phase transformation toughening mechanism and second crystalline phase toughening mechanism of zirconia.

Improved properties of low‐carbon MgO‐C refractories with the addition of multilayer graphene/MgAl2O4 composite powders

AbstractThe paper investigated the effects of different amounts (0%, 3%, 6%, 9%) of in situ multilayer graphene/MgAl2O4 composite powders on the slag resistance, thermal shock resistance, and oxidation resistance of low‐carbon MgO‐C refractories. Comparing with commercial MgAl2O4, the MgAl2O4 in in situ multilayer graphene/MgAl2O4 composite powders has higher lattice strain of crystal, which can trap more Mn and Fe ions, resulting in the better slag resistance. The oxidation decarbonation layer of MgO‐C specimen with 3% composite powders is 9.71 mm, which is lower than not only the specimen with other contents but also specimen containing carbon black/MgAl2O4 powders. Moreover, the residual strength ratio of the specimen C/MA‐3 was 47.47%, which is 28.5% and 8.08% higher than specimens with no additive and with carbon black/MgAl2O4 powders, respectively. Both improving thermal shock and oxidation resistance properties are related with the unique nano structure, multilayer graphene in situ formed between MgAl2O4 grains, of added composite powders. The former is due to higher strain energy consumed by multi‐deflection of cracks inside the multilayer graphene/MgAl2O4 composite powders. And the latter is due to the higher energy of oxidation activation of multilayer graphene/MgAl2O4 composite powders due to effective protection of multilayer graphene by MgAl2O4.

A novel approach to process high-performance lightweight reticulated porous materials

Lightweight reticulated porous materials (LRPMs) with multi-layered struts had been fabricated using vacuum infiltration technique with clay/alumina infiltration slurries. Thermal shock experiments were carried out in order to assess the thermal shock resistance of the LRPMs. Moreover, the residual stresses and thermal stresses within the multi-layered struts were also predicted. The results pointed out that the clay/alumina infiltration slurries composition had a great influence on the thermal shock resistance of the LRPMs. With an increment of clay the residual tensile stresses within the coating layers turned to residual compressive stresses, which were contrary to the results of the matrix. The optimal content of clay in the infiltration slurries for improvement of the LRPMs thermal shock resistance was found to be 30 wt%, the residual strength ratio of the LRPMs reached the max value (87.13%). At the same time, the residual stresses within the coating layer and matrix were 78.64 MPa and 87.71 MPa respectively, which had a positive effect on the inhibition of cracks growth in the multi-layered struts. With a further increase in the clay content, both of the residual stresses and thermal stresses increased gradually, resulting in the deterioration of the LRPMs thermal shock resistance and mechanical properties.

Thermal Shock Resistance of Porous Silicon Carbide Ceramics Prepared Using Clay and Alumina as Additives

ABSTRACTPorous silicon carbide ceramics were prepared by an in situ reaction bonding process using clay and alumina as additives. The effects of alumina additive, pore former on phase composition, microstructure, flexural strength and thermal shock resistance of the ceramics were studied. Thermal shock resistance of porous SiC ceramics due to cooling was evaluated as a function of quenching temperatures and quenching cycles using water- and air-quenching technique. It was observed that residual strength of the quenched samples in water decreased with increase in the quenching temperature but was almost independent of quenching cycles. In water quenching, the surface of the sample cooled almost instantly but the inside remained hot which created an uneven thermal profile and generated microcracks in the sample; as a result sudden reduction of flexural strength was observed. The results showed that flexural strength and thermal shock resistance properties of the ceramics prepared with alumina are better than those of the ceramics prepared without alumina and the material was found suitable for hot gas filtration application.

Itacolumite (Flexible Sandstone) From Kaliana, Charkhi Dadri District, Haryana, India

Abstract The objective of this study is to highlight main causes of flexibility in itacolumite from Kaliana Hills, Charkhi Dadri district, Haryana. Results of petrominerological studies, scanning electron microscopy (SEM), 3D analysis of SEM images, and quantitative micro fabric analysis reveal that the intergranular void spaces are significantly higher in more flexible specimen (mean 47.68 μm) relative to less flexible specimen (mean 14.32 μm, i.e., &amp;lt;3 times), whereas, non-flexible sandstones show negligible intergranular void spaces (4.97-6.40 μm). Therefore, it may be envisaged that the flexibility in Kaliana itacolumite is due to the presence of more intergranular void spaces. The chemical corrosion appears to be the main reason for the formation of more void spaces in between the constituent grains in the flexible sandstones. Leaching effect on the grains is evident on scanning electron microscopic (SEM) images. These solution channels and embayment might have removed the significant amount of cementing material and silica by dissolution and solution activity. Thus, obviously, the flexibility in the itacolumite was induced by solution activity, more the solution activity, more the void spaces and hence more resultant flexibility. Almost vertically dipping beds of sandstone would have facilitated capillary movement and solutioning leading to development of concomitant leaching induced porosity. Higher degree of flexibility in itacolumite specimen from quarry dump, compared to in-situ fresh itacolumite specimen, is apparently due to more exposure of former to surface processes like weathering, more solution activity and corrosion leading to more removal of cementing material and silica accompanied by concomitant augmentation of considerable intergranular void spaces (&amp;gt;3 times), relative to the latter. It is thus clear that relatively less flexibility in in-situ itacolumite is linked with its less solutioning relative to itacolumite from dump. Thermal shock resistance property of itacolumite can be used as a model material to process synthetic flexible ceramics by mixing phases with large differences of thermal expansion properties.

Structural research at high temperature on industrial semi-finished products made from thermostable stainless steels

Austenitic stainless steels possess the best thermal shock resistance properties at low temperatures and the best resistance to elevated temperatures, with versatile demands in the economy.For all stainless steel grades, the best hot workability is obtained when the structure of the steel at the hot-metal working temperature consists of a single phase. Small quantities of ferrite can be tolerated in the structure of the austenitic and martensitic steels as well as small quantities of austenite in the structure of ferritic steels, but these have to be maintained within certain limits, either through an appropriate change of the chemical composition of the steel, or through the control of the hot-metal working temperature.Therefore, the research observes the evolution of the structure during the heating of austenitic stainless steels, stating the soaking zone favorable for reducing the existing content of ferrite down to values that do not worsen the plasticity.Based on the conducted research, a new heating technology was designed, resulting in the reduction of the heating duration and a high efficiency in terms of the dissolution and spheroidization of the residual ferrite in the basic austenitic structure.

High thermal shock resistance realized by Ti/TiH2 doped tungsten-potassium alloys

Tungsten-potassium-titanium (W-K-Ti) alloys were fabricated by spark-plasma-sintering (SPS) and traditional rotary swaging (RS) techniques. Doping conditions and sintering methods were compared on the relative density, microstructure, hardness, recrystallization temperature, and the thermal shock resistance properties. Transient heat loads under 0.37 GW m−2 were applied on the W-K-Ti samples at room temperature for different cycles and pulsed durations. It was found that moderate Ti-doping (0.05 and 0.075 wt%) in W-K alloys can enhance the thermal shock resistance. Low-cycle thermal fatigue test suggested better thermal shock resistance in SPS-sintered W-K-Ti alloy (SPS-W-K-Ti) using TiH2 dopant. The main reason was that the decomposition of TiH2 significantly reduced the oxygen content of the ingots, which improved the ductility and thermal shock resistance of tungsten alloys. Further fracture analyses indicated that Ti-doping through TiH2 formed intra- and inter- Ti-rich voids in TiH2-doped W-K alloy, while only intergranular Ti particles were found lying along grain boundaries in the Ti-doped one. The formation mechanism of the Ti-rich voids was investigated in detail. This work provides a potential route for obtaining plasma facing materials with high thermal shock resistance.

Preparation and properties of porous SiC–Al2O3 ceramics using coal ash

AbstractIn this paper, we first reported that porous SiC–Al2O3 ceramics were prepared from solid waste coal ash, activated carbon, and commercial SiC powder by a carbothermal reduction reaction (CRR) method under Ar atmosphere. The effects of addition amounts of SiC (0, 10, 15, and 20 wt%) on the postsintering properties of as‐prepared porous SiC–Al2O3 ceramics, such as phase composition, microstructure, apparent porosity, bulk density, pore size distribution, compressive strength, thermal shock resistance, and thermal diffusivity have been investigated. It was found that the final products are β‐SiC and α‐Al2O3. Meanwhile, the SEM shows the pores distribute uniformly and the body gradually contacts closely in the porous SiC–Al2O3 ceramics. The properties of as‐prepared porous SiC–Al2O3 ceramics were found to be remarkably improved by adding proper amounts of SiC (10, 15, and 20 wt%). However, further increasing the amount of SiC leads to a decrease in thermal shock resistance and mechanical properties. Porous SiC–Al2O3 ceramics doped with 10 wt% SiC and sintered at 1600°C for 5 hours with the median pore diameter of 4.24 μm, room‐temperature compressive strength of 21.70 MPa, apparent porosity of 48%, and thermal diffusivity of 0.0194 cm2/s were successfully obtained.

Decarburisation behaviour of high-carbon MgO-C refractories in O2-CO2 oxidising atmospheres

High-carbon MgO-C refractories have been widely used in oxygen bottom blowing converters due to their excellent thermal shock resistance property. However, decarburisation is unavoidable in MgO-C refractory materials applied around oxygen bottom blowing tuyeres. To alleviate the decarburisation rate of MgO-C refractories and prolong the furnace life of oxygen bottom blowing converters, a new method consisting of mixing CO2 in the bottom blowing oxygen was proposed by the authors. The decarburisation behaviour of MgO-C refractories in O2-CO2 oxidising atmosphere has not been studied before. Herein, the effects of the CO2 ratio in the O2-CO2 oxidising atmosphere on the decarburisation amount and the microstructure of the decarburisation zone were investigated experimentally. The results show that as the CO2 ratio increases, the weight loss of the MgO-C refractory decreases with a parabolic trend and the decarburisation zone depth decreases with a linear trend. Micrographs show that pores form in the decarburisation zone, and the amount of pores decreases as the CO2 ratio increases. The results confirm that mixing CO2 in the bottom blowing oxygen can effectively alleviate the decarburisation of MgO-C refractories.

On the thermal shock resistance and mechanical properties of novel unidirectional UHTCMCs for extreme environments

Aerospace provides a strong driving force for technological development. Recently a novel class of composites for harsh environments, based on ultra-high temperature ceramic composites reinforced with continuous fibers (UHTCMC), is being developed. The goal of this work is to overcome the current data patchwork about their microstructural optimization and structural behavior, by showing a consistent mechanical characterization of well-defined and developed UHTCMCs based on ZrB2-matrix. The obtained composites have a density of 3.7 g/cm3 and porosity of less than 10%. The flexural strength increased from 360 to 550 MPa from room temperature to 1500 °C, showing a non-brittle behaviour. The composites were able to sustain a thermal shock severity as high as 1500 °C. The maximum decrease of strength at 1400 °C was 16% of the initial value, indicating that the samples could be shocked at even higher temperature. Flexural strength, Young’s modulus and coefficient of thermal expansions (CTE) of the composites were measured both along transverse and longitudinal direction and correlated to the microstructural features. The presented microstructural and mechanical characterization well defines the potentiality of the UHTCMCs and can be used as reference for the design and development of novel thermal protection systems and other structural components for harsh environments.

Optimal design on the high-temperature mechanical properties of porous alumina ceramics based on fractal dimension analysis

Fractal theory and regression analysis were employed for the first time to investigate the effect of pore size and pore distribution on high-temperature mechanical properties of porous alumina ceramics (PAC). In the present work, PAC with the comparable porosity, different pore sizes and pore distributions were prepared using carbon black as the pore-forming agent. Particular emphasis in this study was placed on the establishment of correlation between the thermal shock resistance and pore properties. The relationship between fractal dimension (Df) andthermal shock resistance parameter (Rst) in specimens presented the negative power function, indicating that low Df could benefit the improvement of thermal shock resistance in specimens. The results showed that the increase of pore size and pore sphericity leads to a reduced Df, the enhanced hot modulus of rupture (HMOR) and. The decrease of proportion of micro-pores below 2 μm, the increase of mean pore size and pore sphericity could result in the decrease of Df, and then improve Rst and HMOR of specimens. Based on the correlation between Rst and pore characteristics, PAC with improved thermal shock resistance could be achieved when their pore structure meets the above features.

Fabrication and properties of in situ silicon nitride nanowires reinforced porous silicon nitride (SNNWs/SN) composites

The in situ silicon nitride nanowires reinforced porous silicon nitride (SNNWs/SN) composites were fabricated via gelcasting followed by pressureless sintering. SNNWs were well distributed in the porous silicon nitride matrix. The tip-body appearance suggested a VLS growth mechanism. The flexural strength and elastic modulus of the prepared composites can achieve 84.3 ± 3.9 MPa and 23.3 ± 2.0 GPa respectively (25 °C), while the corresponding porosity was 40.7 vol.%. Remarkably, the strength retention rate of the composites at 1400 °C was up to 66.1%. This is due to the excellent thermal stability of SNNWs and silicon nitride matrix. Also, the fracture toughness of the composites was improved to ∼42% larger than pure porous silicon nitride ceramics because of the bridging effect of the NWs and the interlocking effect of β-Si3N4 crystals. In addition, a good thermal shock resistance and dielectric properties were indicated. The good overall performance made SNNWs/SN composites promising candidate for advanced high-temperature applications.

Raw materials for manufacturing of Superior quality MgO-C bricks

In SAIL Plant, Magnesia carbon (MgO-C) bricks are used as lining material for Metallurgical Vessels like Steel ladles as well as Basic Oxygen Furnaces (BOF). MgO-C bricks are the most suitable refractory in these conditions for slag line as they possess superior slag penetration resistance and excellent thermal shock resistance at elevated temperature. In Iron & Steel Industry, manufacturing processes has changed significantly due to the rising demand of quality steel in the market. Furnace capacities have increased & technological parameters have changed. Plants have been modernized with addition of secondary steel making facilities to meet the stringent quality requirement of newer grades of steel.Refractories in steel ladle are subjected to severe operating conditions like high temperature (1670-1700°C) tapping of liquid steel from BOF, thermal shock, abrasion with molten metal due to stirring during gas purging, localized heating in slag zone due to arcing, corrosion with slag, thermo mechanical stress etc. Hence, use of superior quality refractory material with evenly distributed impurities is essential to get an end product of very high corrosion and spalling resistance and high thermo-mechanical properties. Easier application technique is also required for higher lining life with less down time. Various qualities of magnesia carbon refractories are used in different zones of steel ladles & BOF. Magnesia carbon refractories are essentially constitute of two highly refractory components i.e. magnesia and graphite.Sea water magnesia is mainly used in SAIL as the main raw material for manufacturing of magnesia carbon bricks. Fused magnesia has also been tried by partially mixing with sea water magnesia. Although MgO content in magnesia is important up to a certain level, but the type of impurities and their relative proportions plays vital role in achieving good performance. Besides chemical purity, bulk density and crystal size are considered to be very important properties of magnesia for application in BOF and steel ladles. Beside magnesia, graphite of both indigenous and import sources were used in different occasions. Purity of graphite has been reported to have great influence on slag corrosion resistance and hot properties of magnesia carbon bricks. Particle size distribution of graphite influences porosity and thermal shock resistance properties of magnesia carbon bricks.This paper deals with the properties of different raw materials for making of superior quality MgO-C bricks for improved performance in Metallurgical vessels of SAIL plants.

Thermal shock resistance of pressureless sintered SiC/AlN ceramic composites

This paper conducts for the first time, a study of thermal shock resistance properties of different pressureless sintered SiC/AlN ceramic composites. SiC/AlN composites with different AlN content (0–40 wt%) were obtained by pressureless sintering of SiC and AlN powders at temperature of 2080 °C/2 h in Ar/vacuum atmosphere with using 2.5% yttria and alumina as sintering aids. The different ceramic composites are subjected to 20 thermal shock/water quenching cycles at temperature of 700 °C. Mechanical strength and microstructure investigation before and after thermal shock exposure were used to evaluate thermal shock properties and durability of the different composites. It was found that, the investigated carbide/nitride ceramics gave high thermal shock resistance without any cracking or spalling with retention of about 70% of its original strength. Results suggest that, SiC/AlN composites are strong candidate in severe thermal shock resistance environments.

Research on microstructure and oxidation resistant property of ZrSi2-SiC/SiC coating on HTR graphite spheres

ZrSi2-SiC/SiC coating was prepared on the surface of high temperature gas-cooled reactor (HTR) matrix graphite spheres by two-step pack cementation and sintering process. The microstructure, oxidation resistance and thermal shock resistance properties of the as-prepared coatings with different original powder mixtures were investigated. Results show that dense microstructure of the ZrSi2-SiC/SiC coating and continuous ZrSiO4-SiO2-ZrO2 glass phase generated during the oxidation process were the key factors for the outstanding thermal properties. When the mole ratio of Zr:Si:C reaches 1:7:3 in the second pack cementation powders, the coated graphite spheres have optimum oxidation resistant ability. The weight gain is only 0.6wt% after 15 times thermal shock tests and 0.12wt% after isothermal oxidation test at 1500°C for 20h in air. The oxidation resistant mechanism of the coating was also discussed. The dense inner SiC layer and the outer glass layer generated during the oxidation process could protect the ZrSi2-SiC/SiC coating from further oxidation.